Get the second document
Move to the second sector of the base. From the bunker (M6,11) next to the minefield get on the road and, crawling, go to the door in the rock. The door leads to the central bunker (M6,14).
Inside, there are no more than four enemies - next to the document (M6,15), in the small office, you can find the officer. He is the only one that you must kill if you want to evacuate safely.
Get to the antenna and destroy it
The safest way to the antenna (M6,17) leads from the bunker with the document - leave it and go to the passage (M6,16) on the other side of the road (there are only two enemies). When you get higher you can take care of the antenna and stay unnoticed.
There are two ways to complete this objective. You can destroy the antenna by using explosive charges - to do that you must get on the top of the bunker with the antenna and place the charge. Be careful, there are two snipers and a few guards on the small square below. Additionally, armored reinforcements will arrive after the explosion.
The second, easier, option is to enter the bunker (the entrance is in the south) and sabotage the equipment. You receive the same amount of experience but you won't expose yourself to danger and you won't be discovered.
The last cannon
Finally, best after the sabotage, enter the main square and, if you haven't raised any alarms, the road to the last cannon (M6,18) should be safe. The only difficulty are the snipers (M6,3-5) that surround you from two/three sides. You definitely must eliminate them - they won't allow you to safely set the charges.